3 ms·

> the new one is absolutely indistinguishable from the original This is where I get stuck, because no one explains this. How do we know this is true? Is this b

> the new one is absolutely indistinguishable from the original This is where I get stuck, because no one explains this. How do we know this is true? Is this because they match in all the parameters that we're aware of (say, momentum, spin, whatever), or is this because it has been proven (through a Bell-like test) that there cannot be any other parameters that we are potentially unaware of, and hence if they match in the known ones, then they are one and the same?

sitharus 11y ago
No, it doesn't mean that there is no magical hidden property that actually distinguishes them, but that as far as we can tell they're indistinguishable. Given that nothing about the photons is different they must be the same right? For more confusing theory, look up the single-electron universe.
wfunction 11y ago
Yup I've heard about the single-electron universe. Okay, so doesn't what you said imply there could very well be a hidden variable that we're unaware of, that affects the two entangled photons as they are separated? What's ruling this out?
- sanxiyn 11y ago
  Nothing rules that out. What Bell tests, of which this one is the best implementation so far, rule out is local hidden variable. Hidden variable that can propagate faster than light is not ruled out by the test. (For the most people, it is ruled out by being faster than light.)
Jach 11y ago
The reason we know the photons are indistinguishable is because quantum mechanics deals with configurations of particles at locations, and these configurations predict different outcomes depending on whether "photon A here and photon B there" is a different configuration than "photon B here and photon A there". If A and B are distinguishable, the configurations aren't the same, and you'll notice real quick when your experimental results don't line up with expectations based on them being the same. Mathematically it's as simple as the fact that x^2 + y^2 != (x+y)^2 for all x,y. To read a longer discourse on this, you can check out http://lesswrong.com/lw/ph/can_you_prove_two_particles_are_identical/ http://lesswrong.com/lw/ph/can_you_prove_two_particles_are_i... (and the sequence in general... which can help you with the 'change direction' bits)
- Jach 11y ago
  (Would appreciate if downvoters explained whether they downvoted because my understanding and thus explanation is bunk (hey, I'm not a physicist) or because they're generally against LessWrongthink-peddling...)
jacquesm 11y ago
There could be other parameters but since we're not aware of them to us the photons are indistinguishable. It's in a nutshell why your mirror image still looks remarkably like you. It's all 'brand new' photons moving in a new direction but they carry all the properties of the originals, the only thing that is different is that there are fewer of them because some of the absorbed ones never made it to re-emission because their interaction with the mirror substrate caused them to stay there.

sitharus 11y ago

No, it doesn't mean that there is no magical hidden property that actually distinguishes them, but that as far as we can tell they're indistinguishable. Given that nothing about the photons is different they must be the same right? For more confusing theory, look up the single-electron universe.

wfunction 11y ago

Yup I've heard about the single-electron universe. Okay, so doesn't what you said imply there could very well be a hidden variable that we're unaware of, that affects the two entangled photons as they are separated? What's ruling this out?

sanxiyn 11y ago
Nothing rules that out. What Bell tests, of which this one is the best implementation so far, rule out is local hidden variable. Hidden variable that can propagate faster than light is not ruled out by the test. (For the most people, it is ruled out by being faster than light.)

Jach 11y ago

The reason we know the photons are indistinguishable is because quantum mechanics deals with configurations of particles at locations, and these configurations predict different outcomes depending on whether "photon A here and photon B there" is a different configuration than "photon B here and photon A there". If A and B are distinguishable, the configurations aren't the same, and you'll notice real quick when your experimental results don't line up with expectations based on them being the same. Mathematically it's as simple as the fact that x^2 + y^2 != (x+y)^2 for all x,y. To read a longer discourse on this, you can check out http://lesswrong.com/lw/ph/can_you_prove_two_particles_are_identical/ http://lesswrong.com/lw/ph/can_you_prove_two_particles_are_i... (and the sequence in general... which can help you with the 'change direction' bits)

Jach 11y ago
(Would appreciate if downvoters explained whether they downvoted because my understanding and thus explanation is bunk (hey, I'm not a physicist) or because they're generally against LessWrongthink-peddling...)

jacquesm 11y ago

There could be other parameters but since we're not aware of them to us the photons are indistinguishable. It's in a nutshell why your mirror image still looks remarkably like you. It's all 'brand new' photons moving in a new direction but they carry all the properties of the originals, the only thing that is different is that there are fewer of them because some of the absorbed ones never made it to re-emission because their interaction with the mirror substrate caused them to stay there.